The present invention relates to a hybrid electric vehicle and more particularly to a park pawl for the transmission of such a vehicle.
Series type hybrid electric vehicles have an electric generator driven by a thermal engine to supply electrical power to the vehicles"" battery and electrical power distribution systems, which in turn support operation of an electric drive motor. Unlike parallel type electric vehicles which have a drive line which can be driven directly by the thermal engine, series type electric vehicles are driven by only the electric drive motor. The term series refers to the path of energy from the thermal engines to the drive line and hence to a powered axle and wheels. Several advantages follow from this arrangement. For example, if the drive motor does not require power and the thermal engine is running, then all electrical power can be used to charge the battery, and run other electrical power using systems. Further, the drive motor and thermal engine may be positioned on the vehicle chassis without consideration of other""s position.
In a series type hybrid electric vehicle, the drive motor may be connected to the driven axle through a gear reduction transmission and a drive train. The transmission is typically based on a ring or planetary gear set comprising several rotatable elements. Park brakes have been provided in such vehicles through a mechanical, non-fluid operated mechanism in proximity to the a portion, or portions, of the drive train, such as illustrated in U.S. Pat. No. 6,186,253. However, park pawls, such as commonly found in automatic transmissions, which provide a back up to park brakes, are not readily duplicated with off the shelf motors and with the gear reduction devices used as transmissions. Nor is the possibility of leaving the vehicle in gear to use the thermal engine as a brake available. What is needed is a mechanically reliable park pawl easily implemented with electric vehicles.
According to the invention there is provided a park pawl for a transmission for an electric motor. The transmission comprises gears which may be engaged with the drive motor, to propel a vehicle. The park pawl engages a gear to prevent its rotation relative to the frame of the vehicle. The gear is circular with pawl engagement points around its circumference. The pawl proper comprises a pair of worm gear rollers arrayed on opposite sides of the gear from one another and oriented to have parallel axes of rotation in the plane of the gear and perpendicular to the axis of rotation of the gear. The worm gear rollers have planed faces parallel to their respective axes of rotation with the worm gear rollers being positionable to present the planed faces to the gear and thereby allowing the gear to rotate. The worm gear rollers are further positionable to bring their respective gear threads into engagement with the pawl engagement points of the gear by rotation of the rollers, preventing rotation of the gear.
Additional effects, features and advantages will be apparent in the written description that follows.